I would have a good look through Xero's available fields because I would have expected there to be a field for the cumulative balance in there. It would be better to get that field value from the source than have to construct it via a formula.

Perhaps you could relate the current balance back to the static value for the opening balance. You can maybe use a static cell reference to the opening balance so the formula could be
{=F2-<<Balance>>}
where F2 is the cell reference for the opening balance and every 'cumulative' balance result is basically the difference between the opening balance and the current balance.

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Andrew Lockton
Chrysalis Design, Melbourne Australia